1. Field of the Invention
This invention relates to a polyether urethane elastomer composition having an improved cut-growth and flex-crack resistance and especially having a high fracture resistance at high temperature and at high speed testing condition, and to the process for preparation thereof.
2. Description of the Prior Art
A urethane elastomer is well known to be castable to desired articles under the atmospheric pressure before curing and to have good mechanical properties after curing. Therefore, a urethane elastomer is widely used for preparing rollers, machine parts, solid tires, and the like.
The products made of a polyether urethane elastomer are, however, limited to being used under mild conditions of dynamic deformation, because of the poor cut-growth and flex-crack resistance of a polyether urethane elastomer.
In order to improve this resistance, many attempts have been tried. For example, the nature and the amount of curing agents and the molecular weight of polyether glycol have been studied by many people. However, satisfactory results of these attempts have never been reported. For example, the cut-growth and flex-crack resistance of a polyether urethane elastomer can be improved, to some extent, by increasing the number average molecular weight of the polyether used for a raw material, but such polyether urethane elastomer can not be applied to practical uses since the modulus and the tensile strength at break are extremely low and the permanent set is large.
Our extensive investigation on this problem resulted in the discovery of a new polyether urethane elastomeric composition having improved cut-growth and flex-crack resistance, which was described by us in the specification of U.S. Ser. No. 314,848 filed on Dec. 13, 1972, now U.S. Pat. No. 3,798,200. The specification stated that improved resistance for cut-growth and for flex-cracking can be attained by preparing a polyurethane elastomer by using selected polyether glycol having weight average molecular weight of 4,500 to 20,000, wherein the molecular weight distribution curve of the polyether has at least two peaks and at least one of which is located in the higher molecular weight region and at least another one of which is located in the lower molecular weight region than the rheological critical molecular weight of the polyether. Fracture resistance of this polyether urethane elastomer at high temperature or at high speed testing conditions was, however, found to be inferior in comparison to the conventional polyether urethane elastomer produced by using polyether having a molecular weight distribution curve of a mono-dispersed type. In this connection, the improvement of fracture resistance at high temperature and at high speed testing conditions are desired for urethane elastomer together with the good resistance for cut-growth and for flex-cracking.